Vision systems that perform measurement, inspection, alignment of objects and/or decoding of symbology (e.g. bar codes) are used in a wide range of applications and industries. These systems are based around the use of an image sensor, which acquires images (typically grayscale or color, and in one, two or three dimensions) of the subject or object, and processes these acquired images using an on-board or interconnected vision system processor. The processor generally includes both processing hardware and non-transitory computer-readable program instructions that perform one or more vision system processes to generate a desired output based upon the image's processed information. This image information is typically provided within an array of image pixels each having various colors and/or intensities. In the example of a symbology (barcode) reader, the user or automated process acquires an image of an object that is believed to contain one or more barcodes. The image is processed to identify barcode features, which are then decoded by a decoding process and/or processor obtain the inherent alphanumeric data represented by the code.
Often, a vision system camera includes an internal processor and other components that allow it to act as a standalone unit, providing a desired output data (e.g. decoded symbol information) to a downstream process, such as an inventory tracking computer system. It is desirable that the camera assembly contain a lens mount, such as the commonly used C-mount, that is capable of receiving a variety of lens configurations so that it can be adapted to the specific vision system task. The choice of lens configuration can be driven by a variety of factors driven by such factors as lighting/illumination, field of view, focal distance, relative angle of the camera axis and imaged surface, and the fineness of details on the imaged surface. In addition, the cost of the lens and/or the available space for mounting the vision system can also drive the choice of lens.
An exemplary lens configuration that can be desirable in certain vision system applications is the automatic focusing (auto-focus) assembly. By way of example, an auto-focus lens can be facilitated by a so-called liquid lens assembly. One form of liquid lens uses two iso-density liquids—oil is an insulator while water is a conductor. The variation of voltage passed through the lens by surrounding circuitry leads to a change of curvature of the liquid-liquid interface, which in turn leads to a change of the focal length of the lens. Some significant advantages in the use of a liquid lens are the lens' ruggedness (it is free of mechanical moving parts), its fast response times, its relatively good optical quality, and its low power consumption and size. The use of a liquid lens can desirably simplify installation, setup and maintenance of the vision system by eliminating the need to manually touch the lens. Relative to other autofocus mechanisms, the liquid lens has extremely fast response times. It is also ideal for applications with reading distances that change from object-to-object (surface-to-surface) or during the changeover from the reading of one object to another object.
A recent development in liquid lens technology is available from Optotune AG of Switzerland. This lens utilizes a movable membrane covering a liquid reservoir to vary its focal distance. This lens advantageously provides a larger aperture than competing designs and operates faster. However, due to thermal drift and other factors, the liquid lens may lose calibration and focus over time.
One approach to refocusing a lens after loss of calibration/focus is to drive the lens incrementally to various focal positions and measure the sharpness of an object, such as a runtime object or calibration target. However, this requires time and effort that takes away from runtime operation, and can be an unreliable technique (depending in part on the quality of illumination and contrast of the imaged scene).
It is therefore desirable to provide a system and method for stabilizing the focus of a liquid (or other auto-focusing) lens type that can be employed quickly and at any time during camera operation. This system and method should allow a lens assembly that mounts in a conventional camera base mount and should avoid and significant loss of performance in carrying out vision system tasks. The system and method should allow for focus over a relatively wide range (for example 20 cm to 2 m) of reading distance.